The symbiotic relationship between legume crops and Rhizobium bacteria is well-known. Within the genus, Rhizobium are relatively specific as symbionts for particular legume crops. For example, Rhizobium trifolii is specific in nodulating clover, Rhizobium japonicum for soybeans, etc. However, all of the Rhizobium share a common morphology and characterization as nodulating bacteria for legume crops.
The nodulation mechanisms associated with rhizobia have been extensively studied but are still not fully understood. In soil in which certain crops are grown, such as soybeans, the symbiotic Rhizobium becomes indigenous. If the required Rhizobium is not present in the soil or is there in inadequate amounts, the practice has been to add the selected Rhizobium to the soil, either directly or in admixture with the seeds of the particular legume crop.
Cells of the rhizobia are attracted to the roots of the legume, become attached thereto, and undergo a transformation to a form known as "bacteroids". During the transformation, the rhizobial cells stop dividing and swell to a greatly enlarged size. It is known that the presence of succinate in sufficient concentration can induce swelling of rhizobial cells corresponding to the initial step in forming bacteroids. In 1979 I reported experiments with Rhizobium trifolii wherein succinate induced swelling of the cells to a large, pleomorphic non-dividing form similar to that of bacteroids. Urban (1979) Appl. Environ. Microbiol. 38: 1173-1178. Later experiments showed that a critically high concentration of succinate (viz. 16.6 mM) was inductive for cell swelling of R. trifolii and bacteroid-type transformation. Urban and Dazzo (1982), Appl. Environ. Microbiol. 44: 219-226. These findings suggested that succinate might be involved in the transformation of vegetative rhizobia to the bacteroid morphology of nitrogen-fixing nodules.
Very little has been published with respect to rhizobial mutants. It has been reported that certain R. trifolii mutant strains are incapable of transporting or incorporating C.sub.4 -dicarboxylates (including succinate). R. leguminosarum mutant strains having decreased ability to incorporate succinate tend to form ineffective nodules. Ronson et al. (1981), Proc. Natl. Acad. Sci. U.S.A. 78: 4284-4288; and Glenn and Brewin (1981), J. Gen. Microbiol. 126: 237-241. It has also been reported that succinate dehydrogenase-deficient mutant strains of R. meliloti form ineffective nodules, whereas revertants, like the wild-type strain, form effective nodules on alfalfa. Gardiol (1982) J. Bacteriol. 151: 1621-1623.